Semiconductor device, optical sensor device and semiconductor device manufacturing method

ABSTRACT

Provided are a semiconductor device and an optical sensor device, each having reduced dark current, and detectivity extended toward longer wavelengths in the near-infrared. Further, a method for manufacturing the semiconductor device is provided. The semiconductor device  50  includes an absorption layer  3  of a type II (GaAsSb/InGaAs) MQW structure located on an InP substrate  1 , and an InP contact layer  5  located on the MQW structure. In the MQW structure, a composition x (%) of GaAsSb is not smaller than 44%, a thickness z (nm) thereof is not smaller than 3 nm, and z≧−0.4x+24.6 is satisfied.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation of U.S. patent application Ser. No.13/505,226, filed on Apr. 30, 2012 which is a 371 application ofInternational Application No. PCT/JP2011/061519, filed on May 19, 2011,which claims the benefit of priority of the prior Japanese PatentApplication No. 2010-128162, filed on Jun. 3, 2010, the entire contentsof all of which are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a group III-V semiconductor device anda manufacturing method thereof. More particularly, the invention relatesto: a semiconductor device in which an absorption layer includes a typeII multiple quantum well (hereinafter, referred to as “MQW”) structurehaving detectivity in a long wavelength region of the near-infrared; anoptical sensor device adopting the semiconductor device; and a methodfor manufacturing the semiconductor device.

BACKGROUND ART

Non-Patent Literature 1 discloses a photodiode in which an InGaAs/GaAsSbtype II MQW structure is formed on an InP substrate as a group III-Vcompound semiconductor substrate to realize a cut-off wavelength of 2 μmor more.

Meanwhile, Non-Patent Literature 2 discloses an LED and a laser diode,in which an InGaAs/GaAsSb type II MQW structure is formed as an activelayer on an InP substrate to realize an emission wavelength of 2.14 μm.In the type II MQW structure, GaAsSb has an Sb composition of 34 at. %to 40 at. %, that is, a strain-compensated structure is adopted, inwhich the Sb composition is smaller than the lattice match compositionwith InP. Note that “at. %” is simply referred to as “%” in thefollowing description.

Further, single-phase GaAsSb layers having different Sb compositions aregrown on an InP substrate without forming type II MQW, and PLwavelengths are measured. Then, an InP/GaAsSb type II MQW LED isexamined.

Meanwhile, Patent Literature 1 discloses a semiconductor laser devicehaving a GaInNAsSb quantum well structure. This GaInNAsSb quantum wellstructure is a single quantum well structure (i.e., the number of pairsis 1).

CITATION LIST Patent Literature

-   [PATENT LITERATURE 1] Japanese Laid-Open Patent Publication No.    2005-197395

Non Patent Literature

-   [NON PATENT LITERATURE 1] R. Sidhu, et. al. “A Long-Wavelength    Photodiode on InP Using Lattice-Matched GaInAs—GaAsSb Type-II    Quantum Wells, IEEE Photonics Technology Letters, Vol. 17, No. 12    (2005), pp. 2715 to 2717-   [NON PATENT LITERATURE 2] M. Peter, et. al. “Light-emitting diodes    and laser diodes based on a Ga_(1-x)In_(x)As/GaAs_(1-y)Sb_(y) type    II superlattice on InP substrate” Appl. Phys. Lett., Vol. 74, No. 14    (5 Apr. 1999), pp. 1951 to 1953-   [NON PATENT LITERATURE 3] M. Peter, et. al. “Band gaps and band    offsets in strained GaAs_(1-y)Sb_(y) on InP grown by metalorganic    chemical vapor deposition” Appl. Phys. Lett., Vol. 74, No. 3 (18    Jan. 1999), pp. 410 to 412

SUMMARY OF THE INVENTION Problems to be Solved by the Invention

In the above-mentioned semiconductor photodiode, it is desired to extendthe detectivity toward longer wavelengths, for an increase in theapplication field. For this purpose, it is considered that theInGaAs/GaAsSb type II MQW may have a strain-compensated structure. Insuch an InGaAs/GaAsSb strain-compensated MQW structure, either of InGaAsand GaAsSb that form a pair is caused to generate a compressive stresswhile the other is caused to generate a tensile stress, therebypreventing a strain from occurring in the pair. In order to extend thedetectivity toward longer wavelengths, it is desired that the InGaAs iscaused to generate a tensile stress while the GaAsSb is caused togenerate a compressive stress. In Non-Patent Literature 2, the Sbcomposition of the GaAsSb is lowered so that the GaAsSb is strained bythe InGaAs. In this strain-compensated MQW structure, firstly,degradation of crystallinity is supposed. Therefore, there have been nocases where the strain-compensated MQW structure is used for the purposeof photodetection in which degradation of crystallinity leads directlyto increase in dark current. That is, only the laser diode and the likedisclosed in Non-Patent Literature 2 have been proposed, but there havebeen no cases where the strain-compensated MQW structure is used inphotodiodes.

When forming an MQW structure having 100 to 300 repetitions (pairs), MBE(Molecular Beam Epitaxy) allows instant switching of molecular beams bymeans of a shutter. Therefore, MBE allows automatic switching of valvesunder microcomputer control, and it has been considered that filmdeposition by MBE is almost inevitable for growth of a high-quality MQWstructure having sharp interfaces. In particular, if the problemregarding the crystal growth of a GaAsSb layer that is likely to causephase separation is solely considered, a crystal growth method having ahigh non-equilibriumity is required in order to perform epitaxial growthwhile preventing such phase separation. Therefore, MBE, which is acrystal growth method having a high non-equilibriumity, is suitable.Actually MBE is used for formation of GaAsSb layers in Non-PatentDocument 1.

The above-mentioned semiconductor devices are all targeted tonear-infrared long-wavelength light. In this case, a cap layer or acontact layer located on the surface of the type II MQW structure ispreferably composed of a material that does not absorb thelong-wavelength light. Therefore, InP is often used for the contactlayer. When growing InP by MBE, since solid phosphorus is used as asource, phosphorus (P) is attached to an inner wall of a growth chamber.When the growth chamber is opened for maintenance and then the attachedphosphorus is exposed to the air, the phosphorus is likely to ignite.Therefore, a manufacturing method is adopted, in which an MQW structureis grown by MBE, and an InP contact layer is grown by a growth methodother than MBE. When the manufacturing method is switched from MBE to,for example, OMVPE, an InP wafer as an interim product should be onceexposed to the air. Such exposure to the air causes the InP wafer to becontaminated by impurities. Further, the above-mentioned switching ofthe growth method impedes manufacturing efficiency.

An object of the present invention is to provide: a semiconductor devicehaving reduced dark current, and detectivity extended toward longerwavelengths in the near-infrared; an optical sensor device adopting thesemiconductor device; and a method for manufacturing the semiconductordevice.

Solution to the Problems

A semiconductor device of the present invention is formed on a groupIII-V semiconductor substrate. The semiconductor device includes anabsorption layer of a type II MQW structure, located on the group III-Vsemiconductor substrate. The MQW structure is composed of a repetitionof a layer containing at least Ga, As, and Sb, and a layer containing atleast In, Ga, and As. The layer containing at least Ga, As, and Sb hasan Sb composition x (at. %) not smaller than 44 at. %, and a thickness z(nm) not smaller than 3 nm, and z≧−0.4x+24.6 is satisfied.

The layer containing at least Ga, As, and Sb and the layer containing atleast In, Ga, and As in the MQW structure have substantially the samethickness within a range of variation of ±1.0 nm.

Therefore, in the following description, a thickness z of one layer(e.g., a GaAsSb layer) is reasonably understood as a thickness z of thislayer or the other layer (e.g., an InGaAs layer).

The above invention adopts an increase in the quantum well thickness(factor (F1)) in order to extend the absorption wavelength region towardlonger wavelengths. According to the above configuration, it is possibleto extend the absorbable wavelength range toward longer wavelengthswhile maintaining reduced dark current without increasing the latticedefect density.

The thickness z (nm) of the layer containing at least Ga, As, and Sb isnot smaller than 3 nm, and satisfies z≧−0.4x+24.6 in relation to the Sbcomposition x (≧44 at %) of the layer containing at least Ga, As, andSb. Such an increase in the thickness z is effective for extending theabsorption wavelength region toward longer wavelengths. The x-z rangelimited by z≧−0.4x+24.6 allows the absorbable long-wavelength region tobe 2.4 μm or more. That is, the boundary z=−0.4x+24.6 is the x-z lineformed by the Sb composition x and the thickness z which allow themaximum absorbable wavelength to be 2.43 μm.

The group III-V semiconductor may be InP. Thereby, a type II InPcompound semiconductor MQW structure can be formed by using an InPsubstrate which has been frequently used, and thus a semiconductordevice having detectivity on the long wavelength side in thenear-infrared region can be easily obtained.

The MQW structure is a strain-compensated structure, and the Sbcomposition x of the layer containing at least Ga, As, and Sb and the Incomposition y (at. %) of the layer containing at least In, Ga, and Assatisfy 100≦x+y≦104.

The above invention adopts a strain-compensated structure (factor (F2))in addition to the above-mentioned increase in the quantum wellthickness (factor (F1)). According to the above configuration, in theMQW structure having the strain-compensated structure (factor (F2)), ifthe Sb composition x of the layer containing at least Ga, As, and Sb isincreased while the In composition y of the layer containing at leastIn, Ga, and As is decreased, an energy difference between the valenceband of the layer containing at least Ga, As, and Sb and the conductionband of the layer containing at least In, Ga, and As can be reducedwhile maintaining lattice match with InP (hereinafter, referred to as“InP lattice match”). As a result, the absorbable wavelength region canbe extended toward longer wavelengths while maintaining reduced darkcurrent without increasing the lattice detect density.

The relationship, 100≦x+y≦104, between the Sb composition x of the layercontaining at least Ga, As, and Sb and the In composition y of the layercontaining at least In, Ga, and As is necessary for reducing the latticedefect density with the InP lattice match being maintained whileincreasing the Sb composition x from 44%.

An InP contact layer may be provided on the MQW structure. Thereby, thecontact layer can be composed of a crystalline layer highly permeable tolight on the long wavelength side in the near-infrared region. Further,an InP layer has been successfully used as a contact layer, and isexcellent in surface smoothness, and therefore, has less hygroscopicityand excellent durability.

Preferably, the thickness z (nm) of the layer containing at least Ga,As, and Sb in the MQW structure is smaller than 10 nm (z<10 nm) when theSb composition x is in a range of (a1) 44 at. %≦x≦56.8 at. %, and issmaller than −0.625x+45.5 (z<−0.625x+45.5) when the Sb composition x isin a range of (a2) 56.8 at. %≦x≦68 at. %.

As described above, (F1) an increase in the thickness of each layer inthe MQW structure, and (F2) an increase in the Sb composition of thelayer containing at least Ga, As, and Sb, and a decrease in the Incomposition of the layer containing at least In, Ga, and As(strain-compensated structure), are effective for extending theabsorbable wavelength region. In particular, as for (F1) an increase inthe thickness of each layer in the MQW structure, the thickness z of thelayer containing at least Ga, As, and Sb is preferably controlled asfollows in relation to dark current or detectivity.

In the range of (a1) 44 at. %≦x≦56.8 at. %, it is not very difficult forthe layer containing at least Ga, As, and Sb and the layer containing atleast In, Ga, and As to maintain InP lattice match. Therefore, even ifthe thickness is increased and thereby deviation from precise latticematch is increased, degradation in crystallinity is small. Therefore, byincreasing the thickness, the absorbable wavelength region can beextended toward longer wavelengths while maintaining reduced darkcurrent. However, such increase in the thickness causes reduction inoverlapping of the wave function of holes confined in the valence bandof the layer containing at least Ga, As, and Sb and the wave function ofelectrons confined in the conduction band of the layer containing atleast In, Ga, and As, which might lead to reduction in quantumefficiency and degradation in detectivity. If the thickness z of thelayer containing at least Ga, As, and Sb is 10 nm or more, thedetectivity is significantly reduced. In order to ensure thedetectivity, the thickness z is preferably smaller than 10 nm.

In the range of (a2) 56.8 at. %≦x≦68 at. %, the Sb composition xdeviates toward the higher side from a range of the Sb composition x, inwhich the layer containing at least Ga, As, and Sb solely achieves InPlattice match. In this case, InP lattice match is achieved in the MQWstructure only when the In composition y of the layer containing atleast In, Ga, and As is decreased. Therefore, roughly speaking, thelayer containing at least Ga, As, and Sb does not have the bestcharacteristics of a crystalline layer, but barely maintains InP latticematch. Under such situation, if the thickness z of the layer containingat least Ga, As, and Sb is increased, deviation from the InP latticematch of the layer containing at least Ga, As, and Sb in the MQWstructure becomes apparent (the same holds for the layer containing atleast In, Ga, and As whose thickness is proportionally increased), andthe lattice defect density increases (degrades) with the increase in thethickness z. As a result, dark current increases as the thickness zincreases. Accordingly, in the region of such high Sb composition x, thedark current depends on the thickness z as well as on the Sb compositionx of the layer containing at least Ga, As, and Sb. When the Sbcomposition x and the thickness z are in a range that satisfiesz<−0.625x+45.5, the dark current is in a practically allowable level.

Preferably, the thickness z (nm) of the layer containing at least Ga,As, and Sb in the MQW structure is not greater than 7 nm (z≦7 nm) whenthe Sb composition x is in a range of (b1) 44 at. %≦x≦54.3 at. %, and isnot greater than −0.27x+21.7 (z≦−0.27x+21.7) when the Sb composition xis in a range of (b2) 54.3 at. %≦x≦68 at. %. Thereby, it is possible topursue an increase in detectivity on the long wavelength side in thenear-infrared region, while emphasizing excellent crystallinity andreduced dark current.

Assuming that, in the MQW structure, a lattice mismatch of the layercontaining at least In, Ga, and As is Δω₁ and a lattice mismatch of thelayer containing at least Ga, As, and Sb is Δω₂, a lattice mismatch Δωof the entire MQW structure is defined by Δω={Σ(Δω₁×thickness of thelayer containing at least In, Ga, and As+Δω₂×thickness of the layercontaining at least Ga, As, and Sb}/{Σ(thickness of the layer containingat least In, Ga, and As+thickness of the layer containing at least Ga,As, and Sb)}. Preferably, the Δω is not smaller than −0.2% but notgreater than 0.2%.

Here, it is defined that “if the lattice constant of the substrate is“a” and the lattice constant of the layer containing at least In, Ga,and As (e.g., InGaAs) is “a₁”, the lattice mismatch Δω₁ of the layercontaining at least In, Ga, and As (e.g., InGaAs) isΔω₁={(a−a₁)/a}×100%”. The same holds true for Δω₂. In the followingdescription, the lattice mismatch is the same as above.

In the above configuration, each of the layer containing at least Ga,As, and Sb and the layer containing at least In, Ga, and As cannotsolely satisfy InP lattice match. However, these layers in combinationcan achieve InP lattice match by deviating their lattice constants inopposite directions. Thus, it is possible to reduce the lattice defectdensity, and maintain reduced dark current, while varying the bandstructure so as to be advantageous for extension of the absorptionwavelength region toward longer wavelengths.

The layer containing at least Ga, As, and Sb may be GaAs_(1-x)Sb_(x)(GaAsSb, hereinafter). Thereby, a crystalline layer can be obtained,which is easy to lattice-match with the InP substrate, and hasrelatively high levels of conduction band and valence band which areadvantageous conditions in forming one layer in the type II MQW.

The layer containing at least In, Ga, and As may be In_(y) Ga_(1-y)As(InGaAs, hereinafter). Thereby, a crystalline layer can be obtained,which is easy to lattice-match with the InP substrate, and hasrelatively low levels of conduction band and valence band which areadvantageous conditions in forming the other layer in the type II MQW.

The maximum wavelength at which the absorption layer has detectivity isnot shorter than 2.4 μm. By satisfying the above-mentioned x-yrelationship, an energy difference between the valence band of theGaAsSb and the conduction band of the InGaAs can be reduced to a valuecorresponding to the wavelength of 2.4 μm or less. As a result, thesemiconductor device becomes able to measure substances having anabsorption band at 2.4 μm or more, and thus can be applied to testdevices for a broader range of substances.

Preferably, the semiconductor device has no regrown interface between abottom surface of the absorption layer and an upper surface of asemiconductor layer including the absorption layer and the InP contactlayer.

The regrown interface is an interface between a first crystalline layerand a second crystalline layer which are grown in such a manner that thefirst crystalline layer is grown by a predetermined growth method andexposed to the atmosphere, and thereafter, the second crystalline layeris grown on and in contact with the first crystalline layer by anothergrowth method. Usually, high concentrations of oxygen and carbon aremixed as impurities. Since the semiconductor device of the presentinvention does not have such regrown interface, excellent crystallinitycan be ensured up to the surface of the InP contact layer, therebycontributing to reduction in dark current.

Further, it is possible to manufacture the semiconductor deviceefficiently. That is, as described later, since the layers, from thebuffer layer through the MQW structure to the InP contact layercontaining phosphorus (P), are consistently grown by all metal-organicsource MOVPE, manufacturing of these layers can be continuously executedin the same growth chamber. Further, although the InP contact layercontaining phosphorus is formed, since solid phosphorus is not used as asource, no phosphorus attaches to the inner wall of the growth chamber.Therefore, ignition or the like is not likely to occur at maintenance,resulting in excellent security.

An optical sensor device of the present invention adopts, as aphotodiode, any of the above-mentioned semiconductor devices. Thereby,an optical sensor device can be obtained, which has reduced darkcurrent, and detectivity on the long wavelength side in thenear-infrared region. This optical sensor device may include: a CMOShaving a readout electrode for each pixel of the semiconductor device(photodiode); a spectrometer (diffraction grating); optical elementssuch as a lens; a control device such as a microcomputer; and the like.

A manufacturing method of the present invention manufactures asemiconductor device on a group III-V semiconductor substrate. Thismanufacturing method includes a step of forming an absorption layer of atype II MQW structure on the group III-V semiconductor substrate. TheMQW structure is composed of a layer containing at least Ga, As, and Sb,and a layer containing at least In, Ga, and As. In the MQW formationstep, the Sb composition x (at. %) of the layer containing at least Ga,As, and Sb is not smaller than 44 at. %, and the Sb composition x (at.%) and the thickness z (nm) satisfy z≧−0.4x+24.6.

Further, the group III-V semiconductor substrate may be InP.

The layer containing at least Ga, As, and Sb and the layer containing atleast In, Ga, and As form a strain-compensated structure. When the Sbcomposition x of the layer containing at least Ga, As, and Sb isincreased from 44 at. %, the In composition y (at. %) of the layercontaining at least In, Ga, and As is decreased at a rate of 0.9 to 1.2per increase of 1 at. % of the Sb composition x.

Further, the manufacturing method may include a step of forming an InPcontact layer on the MQW structure.

Preferably, the thickness z (nm) of the layer containing at least Ga,As, and Sb in the MQW structure is not smaller than 3 nm, and is smallerthan 10 nm (z<10 nm) when the Sb composition x is in a range of (a1) 44at. %≦x≦56.8 at. %, and smaller than −0.625x+45.5 (z≦−0.625x+45.5) whenthe Sb composition x is in a range of (a2) 56.8 at. %≦x≦68 at. %.

The semiconductor device manufactured by the above-mentioned method hasreduced dark current, and detectivity extended toward longer wavelengthsin the near-infrared.

Preferably, the thickness z (nm) of the layer containing at least Ga,As, and Sb in the MQW structure is not smaller than 3 nm, and is notgreater than 7 nm (z≦7 nm) when the Sb composition x is in a range of(b1) 44 at. %≦x≦54.3 at. %, and not greater than −0.27x+21.7(z≦−0.27x+21.7) when the Sb composition x is in a range of (b2) 54.3 at.%≦x≦68 at. %. Thereby, it is possible to pursue an increase indetectivity on the long wavelength side in the near-infrared regionwhile ensuring excellent crystallinity.

In the MQW formation step, assuming that a lattice mismatch of the layercontaining at least In, Ga, and As is Δω₁ and a lattice mismatch of thelayer containing at least Ga, As, and Sb is Δω₂, a lattice mismatch Δωof the entire MQW structure is defined by Δω={Σ(Δω₁×thickness of thelayer containing at least In, Ga, and As+Δω₂×thickness of the layercontaining at least Ga, As, and Sb}/{Σ(thickness of the layer containingat least In, Ga, and As+thickness of the layer containing at least Ga,As, and Sb)}. Preferably, the Δω is not smaller than −0.2% but notgreater than 0.2%. Thus, it is possible to maintain the crystallinequality of the MQW structure, and reduce the lattice defect density,while varying the band structures of the layer containing at least Ga,As, and Sb and the layer containing at least In, Ga, and As so as to beadvantageous for extension of the absorption wavelength region towardlonger wavelengths.

The layer containing at least Ga, As, and Sb may be GaAsSb.

Further, the layer containing at least In, Ga, and As may be InGaAs.

Preferably, semiconductor layers including the MQW structure and the InPcontact layer are grown on the InP substrate consistently bymetal-organic vapor phase epitaxy using only meta-organic sources. Themetal-organic vapor phase epitaxy using only metal-organic sources is agrowth method in which metal-organic sources composed of compounds oforganic materials and metals are used as all sources for vapor phaseepitaxy, and it is referred to as “all metal-organic source MOVPE”.

According to the above-mentioned method, the above-mentionedsemiconductor device can be manufactured efficiently. That is, since thelayers up to the InP contact layer containing phosphorus are grownconsistently by all metal-organic source MOVPE, manufacturing of theselayers can be continuously executed in the same growth chamber. Further,even when an InP contact layer containing phosphorus is formed, sincesolid phosphorus is not used as a source, no phosphorus attaches to theinner wall of the growth chamber. Therefore, ignition or the like is notlikely to occur at maintenance, resulting in excellent security.

The all metal-organic source MOVPE has another advantage in that an MQWstructure having sharp heterointerfaces between the respective layerscan be formed. The MQW structure with sharp heterointerfaces allowshighly accurate spectrometry and the like.

In the MQW formation step, preferably, the MQW structure is formed at atemperature not lower than 400° C. but not higher than 560° C. Thereby,an MQW structure having excellent crystallinity is obtained, and darkcurrent can be further reduced. The above “temperature” is the substratesurface temperature monitored by a pyrometer including an IR camera andan IR spectrometer. Accordingly, the substrate surface temperature isexactly the temperature at a surface of an epitaxial layer that is beinggrown on the substrate. Although the above “temperature” has variousnames such as “substrate temperature”, “growth temperature”, “depositiontemperature” and the like, each indicates the monitored temperature.

Advantageous Effects of the Invention

According to the semiconductor device and the like of the presentinvention, it is possible to extend the detectivity toward longerwavelengths in the near-infrared, with the dark current being maintainedat a low level. Further, since the layers from the MQW absorption layerto the InP contact layer are consistently grown by all metal-organicsource MOVPE, high manufacturing efficiency is achieved. Further, sincephosphorus does not attach to the inner wall of the growth chamber,excellent security is achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating a semiconductor device according to afirst embodiment of the present invention. At interfaces 16 and 17, anoxygen concentration and a carbon concentration are each smaller than1×10¹⁷ cm⁻³. A thickness z of a GaAsSb layer is not smaller than 3 nm,and z (nm)≧−0.4x (at. %)+24.6 is satisfied, wherein x is an Sbcomposition of the GaAsSb layer.

FIG. 2 is an enlarged view of an MQW structure shown in FIG. 1.

FIG. 3 is a diagram illustrating a type II (InGaAs/GaAsSb) bandstructure, and a light absorption (reception) phenomenon.

FIG. 4 is a diagram illustrating a range satisfied by an Sb compositionx (%) of GaAsSb and a thickness z (nm) of GaAsSb in a type II(InGaAs/GaAsSb) MQW structure. A line indicates z=−0.625x+45.5, A2 lineindicates z=−0.27x+21.7, and B line indicates z=−0.4x+24.6.

FIG. 5 is a diagram illustrating a piping system and the like of agrowth system for all metal-organic source MOVPE.

FIG. 6A is a diagram illustrating the flow of metal-organic moleculesand the flow of heat.

FIG. 6B is a schematic diagram illustrating the metal-organic moleculesat a substrate surface.

FIG. 7 is a flowchart of a method of manufacturing a photodiode 50 shownin FIG. 1.

FIG. 8 is a diagram illustrating an optical sensor device including aphotodiode array (semiconductor device) according to a second embodimentof the present invention.

FIG. 9 is a diagram illustrating the positions of samples and PLwavelengths thereof, in the relationship between the Sb composition x ofGaAsSb and the thickness z (nm) thereof in an MQW structure according toExample 1. A line indicates z=−0.625x+45.5, A2 line indicatesz=−0.27x+21.7, and B line indicates z=−0.4x+24.6. In FIG. 9, forexample, 2 denotes Comparative Example 2, and encircled 5 denotesExample 5 of the present invention.

FIG. 10 is a diagram illustrating the relationship between thequantum-well-thicknesses of samples in Example 2, and the PL wavelengthsand detectivities thereof.

FIG. 11 is a diagram illustrating the relationship between the Sbcompositions x of GaAsSb in MQW structures of samples in Example 3, andthe PL wavelengths and dark currents (reverse bias voltage Vr=1V)thereof.

DESCRIPTION OF EMBODIMENTS Embodiment 1

FIG. 1 is a diagram illustrating a semiconductor device 50 according toa first embodiment of the present invention. A photodiode 50 includes,on an InP substrate 1, an InP-based semiconductor layered structure(epitaxial wafer) having a configuration described below. While in FIG.1 light is incident on the InP substrate side, light may be incident onthe epitaxial wafer side.

(an InP substrate 1/an InP buffer layer 2/a type II (InGaAs/GaAsSb) MQWabsorption layer 3/an InGaAsdiffusive-concentration-distribution-adjusting layer 4/an InP contactlayer 5)

A p-type region 6 is located so as to reach into the MQW absorptionlayer 3 from the InP contact layer 5. The p-type region 6 is formed byselectively diffusing Zn as a p-type impurity from an opening of aselective diffusion mask pattern 36 that is an SiN film. The SiNselective diffusion mask pattern 36 allows the p-type impurity to bediffused and introduced into the inside of a peripheral part of thephotodiode 50 in a limited manner when viewed in plan. A p-sideelectrode 11 composed of AuZn is disposed on and in ohmic contact withthe p-type region 6, and an n-side electrode 12 composed of AuGeNi isdisposed on and in ohmic contact with the rear surface of the InPsubstrate 1. In this case, the InP substrate 1 is doped with an n-typeimpurity, and has a predetermined level of conductivity. Further, thephotodiode 50 includes an antireflection layer 35 composed of SiON onthe rear surface of the InP substrate 1, so that the photodiode 50 canbe used in such a manner that light is incident on the rear surface ofthe InP substrate. In the type II MQW absorption layer 3, a p-n junction15 is formed at a position corresponding to a boundary front of thep-type region 6. By applying a reverse bias voltage between the p-sideelectrode 11 and the n-side electrode 12, a larger depletion layer isformed on the side where the n-type impurity concentration is lower(n-type impurity background). The background impurity concentration inthe MQW absorption layer 3 is an n-type impurity concentration (carrierconcentration) of about 5×10¹⁵ cm⁻³ or lower. The position of the p-njunction 15 is determined by an intersection between the backgroundimpurity concentration (n-type carrier concentration) of the MQWabsorption layer 3, and the concentration profile of Zn that is a p-typeimpurity. The diffusive-concentration-distribution-adjusting layer 14 isprovided in order to adjust the concentration distribution of the p typeimpurity in the MQW structure constituting the absorption layer 3.However, the diffusive-concentration-distribution-adjusting layer 14 maynot be necessarily formed. In the absorption layer 3, the Znconcentration is preferably 5×10¹⁶ cm⁻³ or lower.

The following will describe the points of the photodiode of the presentembodiment.

(P1) The absorption layer 3 is composed of the type II (InGaAs/GaAsSb)MQW structure, the Sb composition x of the GaAsSb is not smaller than44%, and the Sb composition x of the GaAsSb is increased while the Incomposition y of the InGaAs is decreased so as to absorb the increase ofthe Sb composition x and maintain InP lattice match as a whole. That is,the GaAsSb and the InGaAs form a strain-compensated structure. Thispoint (P1) is identical to factor (F2) that extends the absorptionwavelength region toward longer wavelengths in the near-infrared. Thefactor (F2) will be described later.(P2) The thickness z of a unit quantum well (GaAsSb) is not smaller than3 nm, and satisfies z≧−0.4x (%)+24.6. This feature mainly includes theabove-mentioned factor (F1) an increase in the thickness, and secondaryincludes the factor (F2).

According to the point (P1), an increase in the Sb composition x and adecrease in the In composition y allow reduction in the energydifference between the valence band of the GaAsSb and the conductionband of the InGaAs. In addition, maintaining InP lattice match allowsgrowth of an MQW, a contact layer, and the like having low latticedefect density. As a result, it is possible to extend the absorptionwavelength region toward longer wavelengths while keeping the darkcurrent at a low level.

Further, according to the point (P2), the absorbable long-wavelengthregion can be 2.4 μm or more.

Further, since no regrown interface exists, incorporation of highconcentration oxygen and carbon as impurities is avoided. As a result,excellent crystallinity is maintained up to the surface of the InPcontact layer, resulting in reduced dark current.

<Extension of Absorption Wavelength Region Toward Longer Wavelengths inNear-Infrared>

There are two factors as follows, which extend the absorption wavelengthregion of the type II (GaAsSb/InGaAs) MQW structure toward longerwavelengths in the near-infrared.

(F1) an increase in the thickness of each layer in the MQW structure. Inthe present embodiment, as shown in FIG. 2, the thickness z of theGaAsSb is used as an indicator. Since the thickness of the InGaAs iswithin a range of ±1.0 nm of the thickness z of the GaAsSb, thesethicknesses are regarded to be substantially the same.(F2) an increase in the Sb composition x to 44% or more and a decreasein the In composition in the GaAsSb/InGaAs. The factor (F2) can beregarded as formation of a strain-compensated structure from theviewpoint of InP lattice match. However, from the viewpoint of bandstructure, the factor (F2) is reduction in the energy difference betweenthe valence band of the GaAsSb and the conduction band of the InGaAs. Aweak point of the factor (F2) is securing of InP lattice match orreduction in lattice defect density.

The factor (F1) an increase in the thickness, and the factor (F2)setting of the compositions x and y will be described with reference toFIG. 3. FIG. 3 is a diagram illustrating the type II (InGaAs/GaAsSb)band structure, and light absorption (reception). When light of a longwavelength λ is absorbed, electrons that have occupied the level of thevalence band of the GaAsSb are excited, and the excited electrons occupythe level of the conduction band of the InGaAs. As a result of the lightabsorption, holes are generated in the valence band. In the type II bandstructure, since the electrons are excited from the valence band of theGaAsSb to the conduction band of the InGaAs, the energy difference isreduced as compared with that caused by electron transition from thevalence band to the conduction band in a single-phase structure, therebyrealizing absorption of the long-wavelength light.

When the light of the long wavelength λ is absorbed, the energydifference between the generated holes and electrons is represented byh·(c/λ), where h is Plank's constant (6.626×10⁻³⁴ J·s), and c is thevelocity of light in the medium. In order to extend the absorptionwavelength region toward longer wavelengths, it is necessary to bringthe both ends of an arrow of h·(c/λ) shown in FIG. 3 close to eachother. As shown in FIG. 3, the factor (F2) reduces the energy differenceΔEvc between the valence band of the GaAsSb and the conduction band ofthe InGaAs. That is, the energy difference ΔEvc is reduced by changingthe band structure in which the electron level is formed. Further, it isconsidered that the factor (F1) influences as follows. A single layer inthe MQW structure forms a single well potential. The energy level ofelectrons generated in the well potential tends to increase withreduction in the width (thickness) of the well potential. Such increasein the energy level corresponds to that when particles having wave-likenature, such as electrons, are confined in a small space, the energystate thereof is increased as compared with the case where the particlesare diffused in a large space, and therefore, can be regarded as auniversal nature. If the thickness z is increased, the energy level ofelectrons (holes) shown in FIG. 3 approaches the valence band and theconduction band. As a result, light absorption is achieved even if thewavelength λ is increased and the h·(c/λ) is reduced.

FIG. 4 is a diagram illustrating a range that is defined by the Sbcomposition x (%) of the GaAsSb and the thickness z of the GaAsSb in thetype II InGaAs/GaAsSb MQW structure in the photodiode 50 of the presentembodiment. The In composition y of the InGaAs is set so as to satisfy100≦x+y≦104. The meaning of each boundary line shown in FIG. 4 will bedescribed below. The reason of setting the range shown in FIG. 4 will bedescribed for Example 1 (refer to FIG. 9).

1. B line: z=−0.4x+24.6

The B line defines the composition x and the thickness z, with which themaximum absorbable wavelength is 2.43 μm. In a range where z is on orhigher than the B line, i.e., in an x-z region on or higher than the Bline, the maximum absorption wavelength is 2.4 μm or more. In thepresent invention, in order to achieve the maximum wavelength of 2.4 μmor more, the thickness z is set to be equal to or higher than the Bline.

2. x=44

The Sb composition x=44(%) is a line in which the maximum wavelength of2.3 μm is ensured regardless of the thickness z. When the Sb compositionx is 44% or more, and then if the thickness z is 7 nm or more, themaximum wavelength of 2.3 μm or more is achieved. In the presentembodiment, the Sb composition x is 44(%) or more.

3. z=3

In the present invention, in order to ensure detectivity, the thicknessz is 3 nm or more.

4. A line: z=−0.625x+45.5

This A line defines a boundary beyond which dark current increases. In arange including and outside the A line (in a range where z is on orhigher than the A line), dark current increases, and the S/N ratiodecreases.

In a region inside the A line (in a region where z is positioned underthe A line), the thickness z increases and the Sb composition xincreases as approaching the A line, and thereby the absorptionwavelength region is extended toward longer wavelengths. In the presentembodiment, if low dark current is emphasized, the x-z range can be setinside the A line. As described above, unless both the thickness z andthe Sb composition x approach the A line, it is not possible to realizea semiconductor device in which an absorption wavelength region isextended toward longer wavelengths in the near-infrared, suchsemiconductor device being expected for wide practical applications.

5. A2 line: z=−0.27x+21.7

However, in the case where reduction in dark current is emphasized, inorder to realize excellent crystallinity, it is preferable that thethickness z is set to satisfy z≦7 nm when the Sb composition x is in arange of (b1) 44 at. %≦x≦54.3 at. %, and is set to satisfy z≦−0.27x+21.7(A2 line) when the Sb composition x is in a range of (b2) 54.3 at.%≦x≦68 at. %. The A2 line will be described in detail for Examples.

6. z=10

Detectivity limit is defined by z=10 nm. That is, when the thickness zis 10 nm, overlapping of wave functions of electrons and holes isreduced, which causes reduction in detectivity. That is, the probabilityof transition of electrons from the valence band of the GaAsSb to theconduction band of the InGaAs is lowered, and transition of suchelectrons (absorption) is less likely to occur. This is caused byrequirements in quantum mechanics. If the thickness z is thinner than 10nm, i.e., if z<10 is satisfied, detectivity can be ensured.

7. z=7

However, when the detectivity is emphasized, it is preferable that z≦7(nm) is satisfied.

<MQW Growth Method>

A description will be given of a manufacturing method. An InP substrate1 is prepared. On the InP substrate 1, an InP buffer layer 2, a type II(InGaAs/GaAsSb) MQW absorption layer 3, an InGaAsdiffusive-concentration-distribution-adjusting layer 4, and an InPcontact layer 5 are grown by all metal-organic source MOVPE. In thisembodiment, a method for growing the type II (InGaAs/GaAsSb) MQWabsorption layer 3 will be described in detail.

FIG. 5 illustrates a piping system and the like of a growth system 60for all metal-organic source MOVPE. A quartz tube 65 is placed inside areaction chamber 63, and source gases are introduced into the quartztube 65. In the quartz tube 65, a substrate table 66 is placed rotatablyand hermetically. The substrate table 66 is provided with a heater 66 hfor heating a substrate. The temperature at a surface of a wafer 50 aduring deposition is monitored by an IR temperature monitor 61 through awindow 69 provided at a ceiling of the reaction chamber 63. Themonitored temperature is a temperature which is referred to as atemperature at which growth is performed, or a deposition temperature,or a substrate temperature. When it is described that an MQW structureis formed at a temperature not lower than 400° C. but not higher than560° C. in the manufacturing method of the present invention, thistemperature ranging from 400° C. to 560° C. is a temperature measured bythe temperature monitor. Forced evacuation from the quartz tube 65 isperformed by means of a vacuum pump.

Source gases are supplied through pipes connected to the quartz tube 65.All metal-organic source MOVPE is characterized by that all source gasesare supplied in forms of metal-organic gases. Although source gases ofdopants or the like are not shown in FIG. 5, dopants are also suppliedin forms of metal-organic gases. The metal-organic gases are stored in aconstant temperature bath and kept at a constant temperature. Hydrogen(H₂) and nitrogen (N₂) are used as carrier gases. The metal-organicgases are carried by the carrier gases, and evacuated by the vacuum pumpto be introduced into the quartz tube 65. The amount of the carriergases is precisely controlled by mass flow controllers (MFCs). ManyMFCs, electromagnetic valves, and the like are automatically controlledby a microcomputer.

A method for manufacturing the wafer 50 a will be described. First, ann-type InP buffer layer 2 is epitaxially grown to a thickness of 150 nmon an S-doped n-type InP substrate 1. Tetraethylsilane (TeESi) is usedas an n-type dopant. At this time, trimethylindium (TMIn) andtertiarybutylphosphine (TBP) are used as source gases. The InP bufferlayer 2 may be grown by using phosphine (PH₃) as an inorganic source.Even if the InP buffer layer 2 is grown at a growth temperature of about600° C. or lower, the crystallinity of the underlying InP substrate isnot degraded by the heating at about 600° C. However, when forming anInP contact layer, since the MQW structure including GaAsSb is disposedunder the InP contact layer, the substrate temperature needs to beprecisely maintained within a range of, for example, not lower than 400°C. but not higher than 560° C. The reason is as follows. If the wafer isheated to about 600° C., the GaAsSb is damaged by the heat, and itscrystallinity is significantly degraded. In addition, if the InP contactlayer is formed at a temperature lower than 400° C., the source gasdecomposition efficiency is significantly reduced, and thereby theimpurity concentration in the InP layer is increased. Therefore, ahigh-quality InP contact layer cannot be obtained. Next, an n-dopedInGaAs layer is grown to a thickness of 0.15 μm (150 nm) on the InPbuffer layer 2. This InGaAs layer is also included in the buffer layer 2in FIG. 1.

Next, a type II MQW absorption layer 3 having InGaAs/GaAsSb as a pair ofquantum wells. As described above, in the quantum well structure, theGaAsSb preferably has a thickness z not smaller than 3 nm but smallerthan 10 nm, and the InGaAs preferably has a thickness of z±1.0 nm. Thus,these layers have substantially the same thickness. In FIG. 1, 250 pairsof quantum wells are deposited to form the MQW absorption layer 3. Fordeposition of the GaAsSb, triethylgallium (TEGa), tertiarybutylarsine(TBAs), and trimethylantimony (TMSb) are used. For deposition of theInGaAs, TEGa, TMIn, and TBAs are used. All the source gases areorganic-metal gases, and the molecular weight of each compound is great.Therefore, the source gases are completely decomposed at a relativelylow temperature not lower than 400° C. but not higher than 560° C.,thereby contributing to the crystal growth. The composition change at aninterface of quantum wells can be made sharp by forming the MQWabsorption layer 3 by all metal-organic source MOVPE. As a result,highly accurate spectrophotometry is achieved.

As a source of gallium (Ga), triethylgallium (TEGa) or trimethylgallium(TMGa) may be used. As a source of indium (In), trimethylindium (TMIn)or triethylindium (TEIn) may be used. As a source of arsenic (As),tertiarybutylarsine (TBAs) or trimethylarsenic (TMAs) may be used. As asource of antimony (Sb), trimethylantimony (TMSb) or triethylantimony(TESb) may be used. Alternatively, triisopropylantimony (TIPSb) ortrisdimethylaminoantimony (TDMASb) may be used. By using these sources,a semiconductor device in which an MQW structure has low impurityconcentration and excellent crystallinity can be obtained. As a result,if the semiconductor device is applied to, for example, a photodiode, aphotodiode having reduced dark current and high detectivity can beobtained. Moreover, by using the photodiode, an optical sensor device,such as an imaging device, capable of taking clearer images can berealized.

Next, a description will be given of how the source gases flow duringformation of the MQW structure 3 by all metal-organic source MOVPE. Thesource gases are carried through the pipes, introduced into the quartztube 65, and evacuated. Many kinds of source gases can be supplied tothe quartz tube 65 by increasing the number of pipes. For example, evenif dozen kinds of source gases are used, the source gases are controlledby open/close of the electromagnetic valves.

Flow of each source gas into the quartz tube 65 is turned on/offaccording to open/close of the electromagnetic valve, with the flow rateof the source gas being controlled by the mass flow controller (MFC)shown in FIG. 5. Then, the source gases are forcibly evacuated from thequartz tube 65 by the vacuum pump. The flow of the source gases is notinterrupted but smoothly and automatically conducted. Accordingly,switching of compositions when forming the pairs of quantum wells isquickly performed.

Since the substrate table 66 rotates as shown in FIG. 5, the source gastemperature distribution does not have a directionality such that thesource gas temperature is higher/lower at the source gas inlet side thanat the source gas outlet side. Further, since the wafer 50 a revolves onthe substrate table 66, the flow of the source gas near the surface ofthe wafer 50 a is in a turbulence state. Therefore, even the source gasnear the surface of the wafer 50 a, excluding the source gas contactingthe wafer 50 a, has a great velocity component in the direction of gasflow from the gas inlet side toward the gas outlet side. Accordingly,the heat, which flows from the substrate table 66 through the wafer 50 ato the source gas, is mostly exhausted together with the exhaust gas,constantly. This causes a great temperature gradient or temperature gapin the vertical direction from the wafer 50 a through its surface to thesource gas space.

Further, in the embodiment of the present invention, the substratetemperature is set in a low temperature range of not lower than 400° C.but not higher than 560° C. When all metal-organic source MOVPE usingmetal-organic sources such as TBAs is performed at such a low substratesurface temperature, the decomposition efficiency of the source is high.Therefore, the source gases, which flow in a region very close to thewafer 50 a and contribute to the growth of the MQW structure, arelimited to those efficiently decomposed into a form required for thegrowth.

FIG. 6A is a diagram illustrating the flow of metal-organic moleculesand the flow of heat, and FIG. 6B is a schematic diagram illustratingthe metal-organic molecules at the substrate surface. These figures areused for explaining that setting of the surface temperature is importantin order to obtain sharp composition change at heterointerfaces in theMQW structure.

Although it is assumed that the surface of the wafer 50 a is at themonitored temperature, a sudden temperature drop or a great temperaturegap occurs as described above in the source gas space a little above thewafer surface. Therefore, in the case of using a source gas whosedecomposition temperature is T1° C., the substrate surface temperatureis set at (T1+α), and the α is determined in view of variation intemperature distribution and the like. Under the situation where asudden temperature drop or a great temperature gap occurs from thesurface of the wafer 50 a to the source gas space, if large-sizemetal-organic molecules as shown in FIG. 6B flow against the wafersurface, the compound molecules that are decomposed and contribute tocrystal growth are considered to be limited to those that contact thewafer surface and those in a range equivalent to the thicknesses of afew metal-organic molecules from the wafer surface. Accordingly, asshown in FIG. 6B, it is considered that the metal-organic moleculescontacting the wafer surface and the metal-organic molecules locatedwithin the region equivalent to the thicknesses of a few metal-organicmolecules from the wafer surface mainly contribute to crystal growth,while the metal-organic molecules located outside the region are lesslikely to be decomposed and are evacuated from the quartz tube 65. Whenthe metal-organic molecules near the surface of the wafer 50 a aredecomposed and contribute to crystal growth, the metal-organic moleculeslocated outside the region enter the region as supplemental molecules.

Taking the converse point of view, by setting the wafer surfacetemperature at a temperature slightly higher than the decompositiontemperature of the metal-organic molecules, the range of themetal-organic molecules that can participate in crystal growth can belimited to the thin source gas layer on the surface of the wafer 50 a.

As understood from the above description, when the source gases suitedto the chemical compositions of the above-described pair are introducedby switching the gases using the electromagnetic valves while forciblyevacuating the gases using the vacuum pump, the crystal growth isperformed such that, after a crystal of the previous chemicalcomposition was grown with slight inertia, a crystal of the chemicalcomposition, to which the source gases have been switched, can be grownwithout being affected by the previous source gases. As a result, thecomposition change at the heterointerface can be made sharp. Such asharp composition change means that the previous source gases do notsubstantially remain in the quartz tube 65, and is caused by that thesource gases that flow in the region very close to the wafer 50 a andcontribute to the growth of the MQW structure are limited thoseefficiently decomposed into the form required for the growth (depositionfactor 1). Specifically, as seen from FIG. 5, after one of the twolayers in the quantum well is formed, the source gases for forming theother layer is introduced by opening/closing the electromagnetic valveswhile forcibly evacuating the gases with the vacuum pump. At this time,although some metal-organic molecules that participate in the crystalgrowth with slight inertia remain, the molecules of the one layer thatmay act as supplemental molecules are mostly evacuated and gone. As thewafer surface temperature is set closer to the decomposition temperatureof the metal-organic molecules, the range of the metal-organic moleculesthat participate in the crystal growth (the range from the wafersurface) is reduced.

In the case of forming the MQW structure, if the MQW structure is grownat a temperature of about 600° C., phase separation occurs in the GaAsSblayers in the MQW structure, which makes it impossible to realize aclean and flat crystal growth surface of an MQW structure, and an MQWstructure having excellent periodicity and crystallinity. Therefore, thegrowth temperature is set in the range of not lower than 400° C. but nothigher than 560° C. (deposition factor 2), and all metal-organic sourceMOVPE is adopted as a deposition method for this growth, in whichmetal-organic gases having high decomposition efficiency are used as allsource gases (deposition factor 3). The deposition factor 1significantly depends on the deposition factor 3.

<Semiconductor Device Manufacturing Method>

In the semiconductor device 50 shown in FIG. 1, the InGaAsdiffusive-concentration-distribution-adjusting layer 4 is located on thetype II MQW absorption layer 3, and the InP contact layer 5 is locatedon the InGaAs diffusive-concentration-distribution-adjusting layer 4. Znas a p type impurity is selectively diffused from the opening of theselective diffusion mask pattern 36 disposed on the surface of the InPcontact layer 5, thereby forming the p-type region 6. A p-n junction orp-i junction 15 is formed at an end of the p-type region 6. A reversebias voltage is applied to the p-n junction or p-i junction 15 to form adepletion layer which traps charges caused by photoelectric conversion,and thus the brightness of a pixel is made responsive to the amount ofcharges. The p-type region 6 or the p-n junction (p-i junction) 15 is amain part that constitutes a pixel. The p-side electrode 11 that is inohmic-contact with the p-type region 6 is a pixel electrode, and theamount of charges is read, pixel by pixel, between the p-side electrode11 and the n-side electrode 12 that is set at the ground voltage. On thesurface of the InP contact layer that surrounds the p-type region 6, theselective diffusion mask pattern 36 is left as it is. Further, apassivation layer (not shown) composed of SiON or the like covers theselective diffusion mask pattern 36. The reason why the selectivediffusion mask pattern 36 is left is as follows. After formation of thep-type region 6, if the selective diffusion mask pattern 36 is removedand the wafer is exposed to the atmosphere, a surface level is formed atthe boundary between the surface of the p-type region and the surface ofa region where the mask pattern 36 is removed from the contact layer,which causes an increase in dark current.

It is a point that, after formation of the MQW structure, growth iscontinued in the same deposition chamber or quartz tube 65 by allmetal-organic source MOVPE until the InP contact layer 5 is formed. Inother words, it is a point that no regrown interface is formed becausethe wafer 50 a is not taken out from the deposition chamber beforeformation of the InP contact layer 5 to form the contact layer 5 byanother deposition method. That is, since the InGaAsdiffusive-concentration-distribution-adjusting layer 4 and the InPcontact layer 5 are continuously formed in the quartz tube 65, theinterfaces 16 and 17 are not regrown interfaces. Therefore, the oxygenconcentration and the carbon concentration are both lower than 1×10¹⁷cm⁻³, and no leakage current occurs particularly at a line where thep-type region 6 and the interface 17 intersect.

In the present embodiment, the non-doped InGaAsdiffusive-concentration-distribution-adjusting layer 4 having athickness of, for example, 1.0 μm is formed on the MQW absorption layer3. After formation of the InP contact layer 5, when Zn as a p-typeimpurity is introduced from the InP contact layer 5 to reach the MQWabsorption layer 3 by a selective diffusion method, if thehigh-concentration Zn enters the MQW structure, the crystallinity isdegraded. The InGaAs diffusive-concentration-distribution-adjustinglayer 4 is provided for adjusting the Zn diffusion. The InGaAsdiffusive-concentration-distribution-adjusting layer 4 is notnecessarily provided as described above.

The p-type region 6 is formed by the above-mentioned selectivediffusion, and the p-n junction or p-i junction 15 is formed at an endof the p-type region 6. Even when the InGaAsdiffusive-concentration-distribution-adjusting layer 4 is inserted,since the InGaAs has a small band gap, the electric resistance of thephotodiode can be reduced even if the InGaAs is non-doped. The reducedelectric resistance leads to an increase in responsivity, therebyrealizing a moving picture of high image quality.

Preferably, the undoped InP contact layer 5 is epitaxially grown to athickness of 0.8 μm on the InGaAsdiffusive-concentration-distribution-adjusting layer 4 by allmetal-organic source MOVPE, with the wafer 50 a being placed in the samequartz tube 65. As described above, trimethylindium (TMIn) andtertiarybutylphosphine (TBP) are used as source gases. The use of thesesource gases allows the growth temperature of the InP contact layer 5 tobe not lower than 400° C. but not higher than 560° C., and morepreferably, not higher than 535° C. As a result, the GaAsSb in the MQWstructure located under the InP contact layer 5 is not damaged by heat,and the crystallinity of the MQW structure is not degraded. When formingthe InP contact layer 5, since the MQW structure including GaAsSb isdisposed under the InP contact layer 5, the substrate temperature shouldbe precisely maintained within a range of not lower than 400° C. but nothigher than 560° C. The reason is as follows. If the wafer is heated toabout 600° C., the GaAsSb is damaged by the heat, and its crystallinityis significantly degraded. In addition, if the InP contact layer isformed at a temperature lower than 400° C., the source gas decompositionefficiency is significantly reduced, and thereby the impurityconcentration in the InP contact layer 5 is increased. Therefore, ahigh-quality InP contact layer 5 cannot be obtained.

As described above, conventionally, a MQW structure has to be formed byMBE. However, growth of an InP contact layer by MBE requires a solidsource as a source of phosphorus, which leads to a problem in terms ofsafety or the like. Further, there is still a room for improvement interms of manufacturing efficiency.

Prior to the present invention, the interface between the InGaAsdiffusive-concentration-distribution-adjusting layer and the InP contactlayer was a regrown interface once exposed to the atmosphere. Theregrown interface can be identified when it satisfies either of theoxygen concentration of 1×10¹⁷ cm⁻³ or more and the carbon concentrationof 1×10¹⁷ cm⁻³ or more, which concentrations can be confirmed bysecondary ion mass spectrometry. The regrown interface and the p-typeregion form an intersection line, and leakage current occurs in theintersection line. Such leakage current significantly degrades the imagequality.

Further, if the InP contact layer is grown by simple MOVPE, sincephosphine (PH₃) is used as a source of phosphorus, the decompositiontemperature is high. Such high decomposition temperature induces athermal damage of the underlying GaAsSb, and thus the crystallinity ofthe MQW structure is degraded.

FIG. 7 is a flowchart of a method of manufacturing the photodiode 50shown in FIG. 1. According to the manufacturing method, it is importantto decrease the growth temperature (deposition factor 2) by using onlymetal-organic gases as source gases (deposition factor 3), and to avoidformation of a regrown interface (deposition factor 4) by growing thelayers consistently in the same deposition chamber or quartz tube 65until formation of the InP contact layer 5 is completed. Thereby,photodiodes having less leakage current, excellent crystallinity, anddetectivity in a wavelength region of 2 μm to 5 μm can be efficientlymanufactured in large numbers.

Embodiment 2

FIG. 8 illustrates an optical sensor device 10 including a photodiodearray (semiconductor device) 50 according to a second embodiment of thepresent invention. In FIG. 8, optical components such as lenses areomitted. Although a passivation layer 43 composed of an SiON layer isshown in FIG. 8, this passivation layer is actually provided in FIG. 1.The photodiode array 50 has the same layered structure as that of thephotodiode shown in FIG. 1. The photodiode array 50 comprises aplurality of photodiodes or pixels P. Since the thickness z and the Sbcomposition x of the photodiode array 50 are identical to those of thesemiconductor device shown in FIG. 1, repeated description is notnecessary. Further, like the photodiode (semiconductor device) shown inFIG. 1, the interfaces 16 and 17 are not regrown interfaces, and theimpurity concentrations of oxygen, carbon, and the like are low.

In FIG. 8, the photodiode array 50 and a CMOS 70 constituting a readoutIC are connected. Readout electrodes (not shown) of the CMOS 70 andpixel electrodes (p-side electrodes) 11 of the photodiode array 50 areconnected via interconnection bumps 39. A ground electrode (n-sideelectrode) 12 common to the pixels of the photodiode array 50, and aground electrode (not shown) of the CMOS 70 are connected via a bump 12b. It is possible to realize an imaging device or the like by combiningthe CMOS 70 and the photodiode array 50, and integrating, pixel bypixel, light absorption information.

As described above, the photodiode array (semiconductor device) 50 ofthe present invention has detectivity in the long wavelength region, andreduced dark current (leakage current). Therefore, when applied tobiological tests for animals and plants, environmental monitoring, andthe like, the photodiode array (semiconductor device) 50 can executehighly precise examination.

EXAMPLES Example 1

Samples were prepared by varying the thickness z (nm) and the Sbcomposition x (%) of GaAsSb in the MQW structure of the photodiode 50having the same structure as the semiconductor device 50 shown in FIG.1, and the PL (Photo-Luminescent) wavelength and the dark current ofeach sample were measured at room temperature. Table I shows the Sbcomposition x of GaAsSb, the In composition y of InGaAs, and thethickness z (nm) of GaAsSb and InGaAs of each sample. The GaAsSb and theInGaAs have the same thickness. Further, assuming that the latticemismatch of the InGaAs is Δω₁ and the lattice mismatch of the GaAsSb isΔω₂, the lattice mismatch Δω of the entire MQW structure is defined byΔω={Σ(Δω₁×InGaAs thickness+Δω₂×GaAsSb thickness)}/{Σ(InGaAsthickness+GaAsSb thickness)}. The composition x and the composition yare set so that the lattice mismatch Δω is not smaller than −0.2% butnot greater than 0.2%.

Each photodiode 50 was manufactured by all metal-organic source MOVPE asdescribed above. The part other than the absorption layer wasmanufactured by the above-mentioned method. The major epitaxial layerswere manufactured as follows. The growth temperature was 510° C. As forthe GaAsSb, TEGa, TBAs, and TESb were used as sources, and the Sbcomposition x was varied by varying the supply ratio of TBAs to TESbwith the V/III ratio (supply of group V sources/supply of group IIIsources) being kept constant. As for the InGaAs, TEGa, TMIn, and TBAswere used as sources, and the In composition y was varied by varying thesupply ratio of TEGa to TMIn with the VIII ratio being kept constant.Further, as for the InP contact layer 5, TMIn and TBP were used assources.

TABLE I Sb In compo- compo- Thick- PL Dark current sition sition nesswavelength (nA) of x(at. y(at. z(nm) of (nm) at photodiode %) of %) ofInGaAs, room temp- 100 μm dia. Sample GaAsSb InGaAs GaAsSb erature Vr =1 V Invention 44.2 58 7 2490 50 Example 1 Comparative 44.1 58.3 5 221030 Example 2 Comparative 46.3 56 5 2290 30 Example 3 Invention 49 53 102870 100 Example 4 Invention 48.6 53 7 2700 80 Example 5 Invention 49.153 6 2620 50 Example 6 Invention 49 53 5 2430 40 Example 7 Comparative49 53.2 3.5 2210 20 Example 8 Invention 51.8 49.7 5 2520 70 Example 9Comparative 52.2 49.8 3.5 2300 50 Example 10 Invention 54.3 47.2 7 2860200 Example 11 Invention 54 47.4 5 2630 100 Example 12 Invention 54.1 473.5 2410 80 Example 13 Invention 58 43.2 5 2790 200 Example 14 Invention57.7 43 3.5 2580 150 Example 15 Invention 61.6 37.7 7 3160 700 Example16 Invention 61.8 38 5 2980 300 Example 17 Invention 62 38.1 3.5 2770200 Example 18 Invention 64.8 35.2 5 3100 800 Example 19

<Evaluation>

The PL wavelength at room temperature, and the dark current weremeasured as follows. The dark current was measured under the conditionthat the absorption diameter was 100 μm, and the reverse bias voltage(Vr) was 1V.

1. PL Wavelength at Room Temperature

The PL wavelength at room temperature corresponds to the band gapwavelength, and corresponds to the maximum wavelength that can be almostabsorbed. With reference to FIGS. 9 and 4, in Comparative Examples 2, 3,8 and 10, the thicknesses z thereof are positioned under the B line:−0.4x+24.6. According to Table I, the PL wavelengths thereof are 2210nm, 2290 nm, 2210 nm and 2300 nm, respectively, which are smaller than2400 nm.

This is because the thicknesses z and the Sb compositions x of GaAsSbare not sufficiently large.

In contrast, in a range where the thickness z is not smaller than 3 nm(z≧3) and is not smaller than −0.4x+24.6 (z≧−0.4x+24.6), i.e., on orhigher than the B line, as shown in Table I, all Invention Examplesachieve the PL wavelengths longer than 2.4 μm. Furthermore, as thethickness z approaches the A line: z=−0.625x+45.5, the PL wavelengthshifts toward longer wavelengths. In FIG. 9, it is indicated by arrowsand PL wavelengths that the PL wavelengths increase as the thickness zapproaches the A line and the line of z=10.

Further, as the thickness z approaches the line of z=10, the PLwavelength increases in the order of Invention Example 7 (2430nm)→Invention Example 6 (2620 nm)→Invention Example 5 (2700nm)→Invention Example 4 (2870 nm).

2. Dark Current

The dark current is limited by the A line: z=−0.625x+45.5. Since thedark current increases with an increase in the lattice defect density,it is often considered that the dark current is caused by only the Sbcomposition x and the In composition y. However, as described above, thedark current also depends on the thickness, and is influenced by notonly the compositions x and y but by the thickness z. This is becausethe lattice defect is accumulated.

Invention Examples 16 and 19 are positioned on the A line, and have darkcurrents of 700 nA and 800 nA, respectively. This level of dark currentis allowable in many applications. However, when the dark current isemphasized, the thickness z is preferably lower than the A line. Inorder to further reduce the dark current with reliability, the thicknessz should be on or lower than the A2 line: z=−0.27x+21.7. In this case,Invention Examples 16 and 19 are outside the scope of the presentinvention, and are regarded as Comparative Examples.

In a range a little apart from the A line, for example, InventionExamples 11, 14, 17 and 18 have dark currents of 200 nA, 200 nA, 300 nAand 200 nA, respectively, which cause no problem in practicalapplications.

3. Detectivity

Although it will be later described for Example 2, if the thickness z ofthe GaAsSb reaches 10 nm, the detectivity degrades. Since a certainlevel of detectivity is achieved even when the thickness z is 10 nm,this thickness z is allowed in some applications. However, if thedetectivity is significantly concerned, the thickness z should besmaller than 10 nm. If higher detectivity is desired, z≦7 (nm) should besatisfied. In this case, Invention Example 4 is outside the scope of thepresent invention.

This is a quantum-mechanical effect, and is true regardless of theconditions of Examples. Also when the thickness z is as small as 2 nm,the detectivity is low. Therefore, the thickness z is 3 nm or more.

Example 2 Manufacturing of Samples

On an S-doped InP substrate, epitaxial layers including a buffer layer(InP/InGaAs), a type II (InGaAs/GaAsSb) MQW absorption layer, an InGaAsdiffusive-concentration-distribution-adjusting layer, and an InP contactlayer were consistently formed by all metal-organic source MOVPE. An InPcontact layer was grown directly on the MQW absorption layer 3. TEGa,TMIn, TBAs, TBP and TMSb were used as sources of Ga, In, As, P and Sb,respectively. TeESi was used as an n-type impurity dopant.

Specifically, an n-doped InP buffer layer was grown to a thickness of150 nm on an S-doped InP substrate, and an n-doped InGaAs buffer layerwas grown to a thickness of 0.15 μm on the n-doped InP buffer layer. Onthe two buffer layers, a type II (InGaAs/GaAsSb) MQW absorption layerwas grown. In the MQW structure, a lower-side non-doped InGaAs layer anda non-doped GaAsSb layer were paired, and 250 pairs were repeated. Thecomposition was determined so that each layer solely achieves latticematch. The Sb composition x of the GaAsSb was 49%, and the Incomposition y of the InGaAs was 53%. Assuming that the lattice mismatchof the InGaAs is Δω₁ and the lattice mismatch of the GaAsSb is Δω₂, thelattice mismatch Δω of the entire MQW structure is defined byΔω={Σ(Δω₁×InGaAs thickness+Δω₂×GaAsSb thickness)}/{Σ(InGaAsthickness+GaAsSb thickness)}. The lattice mismatch Δω is not smallerthan −0.2% but not greater than 0.2%. An InGaAsdiffusive-concentration-distribution-adjusting layer was grown to athickness of 1.0 μm on the MQW absorption layer, and a non-doped InPcontact layer was grown to a thickness of 0.8 μm on thediffusive-concentration-distribution-adjusting layer. For the growth ofthe GaAsSb, TEGa, TBAs and TMSb were used as sources. For the growth ofthe InGaAs, TEBa, TMIn and TBAs were used. Further, for the growth ofthe InP contact layer and the InP buffer layer, TMIn and TBP were used.

TABLE II Sample Comparative Comparative Invention Invention InventionExample A1 Exame A2 Example A3 Example A4 Example A5 Structure Thicknessz(nm) of 2 3 5 7 10 InGaAs, GaAsSb Evaluation PL peak wavelength 1.9 2.12.5 2.7 2.9 (μm) at room temperature Detectivity (A/W) 0.1 0.6 0.6 0.50.2 of photodiode 1 mm dia. Vr = 5 V λ = 2000 nm

<Evaluation>

1. PL Characteristics

Results are shown in Table II and FIG. 10. When the thickness z of theGaAsSb and the thickness of the InGaAs were 5 nm, the PL peak wavelengthwas 2.5 μm. As the thicknesses of both layers were decreased, the PLwavelength shifted toward the shorter wavelengths. When the thicknessesof the GaAsSb and the InGaAs were 2 nm, the PL wavelength was 1.9 μm.

On the other hand, the PL wavelength increased with an increase in thethicknesses of the InGaAs and the GaAsSb. When the thicknesses of bothlayers were 10 nm, the PL wavelength was 2.9 μm.

2. Detectivity

Detectivity measurement was performed for light having a wavelength of2000 nm, at a reverse bias voltage Vr=5V. When the thicknesses of theGaAsSb and the InGaAs were 2 nm, the detectivity was as very low as 0.1A/W. When the thicknesses of both layers were increased to 3 nm, 5 nm,and 7 nm, the detectivity was improved to 0.6 A/W, 0.6 A/W, and 0.5 A/W,respectively. When the thicknesses of both layers were 10 nm, thedetectivity was degraded to 0.2 A/W.

Example 3

Samples having the structure of Example 2 were manufactured according tothe same procedure. However, in Example 3, the thicknesses of the GaAsSband the InGaAs were fixed to 5 nm, and the supply of source gases wascontrolled as described above. Assuming that the lattice mismatch of theInGaAs is Δω₁ and the lattice mismatch of the GaAsSb is Δω₂, the latticemismatch Δω of the entire MQW structure is defined by Δω={Σ(Δω₁×InGaAsthickness+Δω₂×GaAsSb thickness)}/{Σ(InGaAs thickness+GaAsSb thickness)}.The lattice mismatch Δω is not smaller than −0.2% but not greater than0.2%.

TABLE III Sample Invention Invention Invention Invention InventionExample B1 Example B2 Example B3 Example B4 Example B5 Structure Incomposition 53 50 47 43 38 y(at. %) of InGaAs Sb composition 49 52 54 5862 x(at. %) of GaAsSb Evaluation PL peak wavelength   2.4   2.5   2.6  2.8   3.0 (μm) at room temperature Dark current (nA) 40 70 100 200 300of photodiode (excellent (excellent (excellent (excellent (excellent 100μm dia. character- character- character- character- character- Vr = 1 Vistics) istics) istics) istics) istics) Dark current (nA) 50 100 200 500800 of photodiode (excellent (excellent (excellent (excellent (excellent100 μm dia. character- character- character- character- character- Vr =5 V istics) istics) istics) istics) istics)

<Evaluation>

1. PL Characteristics

Results are shown in Table III and FIG. 11. When the Sb composition x ofthe GaAsSb was 49% and the In composition y of the InGaAs was 53%, thePL wavelength was 2.4 μm. When the Sb composition x of the GaAsSb wasincreased and the In composition y of the InGaAs was decreased, the PLwavelength shifted toward longer wavelengths.

When the Sb composition x was 62% and the In composition y was 38%, thePL wavelength was 3.0 μm.

2. Dark Current

Regardless of whether the reverse bias voltage Vr was 1V or 5V, when theSb composition was increased from 44%, the dark current was graduallyincreased. However, this dark current was at a favorable level. When theIn composition y was lower than 53% and the Sb composition was increasedfrom 62%, a difference between the dark current at the reverse biasvoltage Vr of 1V and the dark current at the reverse bias voltage Vr of5V was increased. Therefore, in order to increase the S/N ratio in aphotodiode having the detectivity in the longer wavelength region, it isdesirable that the reverse bias voltage is reduced (the absolute valuethereof is reduced).

Embodiments and Examples of the present invention have been describedabove. However, the embodiments and the examples of the presentinvention disclosed above are only illustrative, and the scope of thepresent invention is not limited to the specific embodiments of theinvention. It is to be understood that the scope of the presentinvention is defined in the appended claims and includes equivalence ofthe description of the claims and all changes within the scope of theclaims.

INDUSTRIAL APPLICABILITY

According to the semiconductor device of the present invention, it ispossible to extend the detectivity toward longer wavelengths in thenear-infrared region by increasing the Sb composition (decreasing the Incomposition) and increasing the thickness of each quantum well, whilereducing the dark current to a level that causes no problem in practicalapplications, and while controlling the Sb and In compositions so thatthe InP lattice match conditions are satisfied by both the GaAsSb andthe InGaAs in combination. As a result, the semiconductor device becomesapplicable to important applications. Further, by growing the layersconsistently in the same growth chamber by all metal-organic sourceMOVPE, contamination due to impurities is avoided, resulting inhigh-quality crystallinity. Further, the sharpness of the compositionbetween the quantum wells in the MQW absorption layer can be increased,which enables highly-precise execution of absorption spectrum analysisor the like.

DESCRIPTION OF THE REFERENCE CHARACTERS

-   -   1 InP substrate    -   2 buffer layer (InP and/or InGaAs)    -   3 type II MQW absorption layer    -   4 InGaAs layer (diffusive-concentration-distribution-adjusting        layer)    -   5 InP contact layer    -   6 p-type region    -   10 optical sensor device (detection device)    -   11 p-side electrode (pixel electrode)    -   12 ground electrode (n-side electrode)    -   12 b bump    -   15 p-n junction    -   16 interface between MQW and InGaAs layer    -   17 interface between InGaAs layer and InP contact layer    -   35 AR (Anti-Reflection) layer    -   36 selective diffusion mask pattern    -   39 interconnection bump    -   43 passivation layer (SiON layer)    -   50 photodiode (photodiode array)    -   50 a wafer (interim product)    -   60 growth system for all metal-organic source MOVPE    -   61 IR temperature monitor    -   63 reaction chamber    -   65 quartz tube    -   69 window of reaction chamber    -   66 substrate table    -   66 h heater    -   70 CMOS    -   P pixel

The invention claimed is:
 1. A semiconductor device formed on an InPsubstrate, comprising: an absorption layer of a type II multiple quantumwell structure, located on the InP substrate, wherein the multiplequantum well structure is a strain-compensated structure composed of arepetition of a GaAs_(1-x)Sb_(x) layer and an In_(y)Ga_(1-y)As layer,and the GaAs_(1-x)Sb_(x) layer has an Sb composition x (at. %) and athickness z (nm) which satisfy relationships (1) to (2) below:44 at. %≦x≦54.3 at. %, and  (1)7 nm≦z<10 nm,  (2) wherein in a maximum wavelength at which theabsorption layer has detectivity is not shorter than 2.4 μm, and whereinthe Sb composition x of the GaAs_(1-x)Sb_(x) layer and an In compositiony (at. %) of the In_(x)Ga_(1-y)As layer satisfy 100≦x+y≦104.
 2. Thesemiconductor device according to claim 1, including an InP contactlayer on the multiple quantum well structure.
 3. The semiconductordevice according to claim 1, wherein when, in the multiple quantum wellstructure, a lattice mismatch of the In_(y)Ga_(1-y)As layer is Δω₁ and alattice mismatch of the GaAs_(1-x)Sb_(x) layer is Δω₂, a latticemismatch Δω of the entire multiple quantum well structure is defined byΔω={Σ(Δω₁×thickness of the In_(y)Ga_(1-y)As layer+Δω₂×thickness of theGaAs_(1-x)Sb_(x) layer)}/{Σ(thickness of the In_(y)Ga_(1-y)Aslayer+thickness of the GaAs_(1-x)Sb_(x) layer)}, and the Δω is notsmaller than −0.2% but not greater than 0.2%.
 4. The semiconductordevice according to claim 3, wherein there is no regrown interfacebetween a bottom surface of the absorption layer and an upper surface ofa semiconductor layer including the absorption layer and the InP contactlayer.
 5. An optical sensor device adopting, as a photodiode, thesemiconductor device according to claim
 1. 6. A method of manufacturinga semiconductor device on an InP substrate, comprising: a step offorming an absorption layer of a type II multiple quantum well structureon the InP substrate, wherein the multiple quantum well structure is astrain-compensated structure composed of a GaAs_(1-x)Sb_(x) layer and anIn_(y)Ga_(1-y)As layer, and in the step of forming the multiple quantumwell structure, an Sb composition x (at. %) and a thickness z (nm) ofthe GaAs_(1-x)Sb_(x) layer satisfy relationships (1) to (2) below:44 at. %≦x≦54.3 at. %, and  (1)7 nm≦z<10 nm,  (2) wherein in a maximum wavelength at which theabsorption layer has detectivity is not shorter than 2.4 μm, and whereinthe Sb composition x of the GaAs_(1-x)Sb_(x) layer and an In compositiony (at. %) of the In_(y)GaAs_(1-y) layer satisfy 100≦x+y≦104.
 7. Themethod of manufacturing a semiconductor device according to claim 6,wherein an In composition y (at. %) of the In_(y)Ga_(1-y)As layer isdecreased at a rate of 0.9 to 1.2 per increase of 1 at. % of the Sbcomposition x of the GaAs_(1-x)Sb_(x) layer.
 8. The method ofmanufacturing a semiconductor device according to claim 6, wherein inthe step of forming the multiple quantum well structure, the multiplequantum well structure is formed at a temperature not lower than 400° C.but not higher than 560° C.